1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Kaleidoscope: Extending the Language: Mutable Variables / SSA
construction</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="Chris Lattner">
<meta name="author" content="Erick Tryzelaar">
<link rel="stylesheet" href="../llvm.css" type="text/css">
</head>
<body>
<div class="doc_title">Kaleidoscope: Extending the Language: Mutable Variables</div>
<ul>
<li><a href="index.html">Up to Tutorial Index</a></li>
<li>Chapter 7
<ol>
<li><a href="#intro">Chapter 7 Introduction</a></li>
<li><a href="#why">Why is this a hard problem?</a></li>
<li><a href="#memory">Memory in LLVM</a></li>
<li><a href="#kalvars">Mutable Variables in Kaleidoscope</a></li>
<li><a href="#adjustments">Adjusting Existing Variables for
Mutation</a></li>
<li><a href="#assignment">New Assignment Operator</a></li>
<li><a href="#localvars">User-defined Local Variables</a></li>
<li><a href="#code">Full Code Listing</a></li>
</ol>
</li>
<li><a href="LangImpl8.html">Chapter 8</a>: Conclusion and other useful LLVM
tidbits</li>
</ul>
<div class="doc_author">
<p>
Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="intro">Chapter 7 Introduction</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Welcome to Chapter 7 of the "<a href="index.html">Implementing a language
with LLVM</a>" tutorial. In chapters 1 through 6, we've built a very
respectable, albeit simple, <a
href="http://en.wikipedia.org/wiki/Functional_programming">functional
programming language</a>. In our journey, we learned some parsing techniques,
how to build and represent an AST, how to build LLVM IR, and how to optimize
the resultant code as well as JIT compile it.</p>
<p>While Kaleidoscope is interesting as a functional language, the fact that it
is functional makes it "too easy" to generate LLVM IR for it. In particular, a
functional language makes it very easy to build LLVM IR directly in <a
href="http://en.wikipedia.org/wiki/Static_single_assignment_form">SSA form</a>.
Since LLVM requires that the input code be in SSA form, this is a very nice
property and it is often unclear to newcomers how to generate code for an
imperative language with mutable variables.</p>
<p>The short (and happy) summary of this chapter is that there is no need for
your front-end to build SSA form: LLVM provides highly tuned and well tested
support for this, though the way it works is a bit unexpected for some.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="why">Why is this a hard problem?</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
To understand why mutable variables cause complexities in SSA construction,
consider this extremely simple C example:
</p>
<div class="doc_code">
<pre>
int G, H;
int test(_Bool Condition) {
int X;
if (Condition)
X = G;
else
X = H;
return X;
}
</pre>
</div>
<p>In this case, we have the variable "X", whose value depends on the path
executed in the program. Because there are two different possible values for X
before the return instruction, a PHI node is inserted to merge the two values.
The LLVM IR that we want for this example looks like this:</p>
<div class="doc_code">
<pre>
@G = weak global i32 0 ; type of @G is i32*
@H = weak global i32 0 ; type of @H is i32*
define i32 @test(i1 %Condition) {
entry:
br i1 %Condition, label %cond_true, label %cond_false
cond_true:
%X.0 = load i32* @G
br label %cond_next
cond_false:
%X.1 = load i32* @H
br label %cond_next
cond_next:
%X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
ret i32 %X.2
}
</pre>
</div>
<p>In this example, the loads from the G and H global variables are explicit in
the LLVM IR, and they live in the then/else branches of the if statement
(cond_true/cond_false). In order to merge the incoming values, the X.2 phi node
in the cond_next block selects the right value to use based on where control
flow is coming from: if control flow comes from the cond_false block, X.2 gets
the value of X.1. Alternatively, if control flow comes from cond_true, it gets
the value of X.0. The intent of this chapter is not to explain the details of
SSA form. For more information, see one of the many <a
href="http://en.wikipedia.org/wiki/Static_single_assignment_form">online
references</a>.</p>
<p>The question for this article is "who places the phi nodes when lowering
assignments to mutable variables?". The issue here is that LLVM
<em>requires</em> that its IR be in SSA form: there is no "non-ssa" mode for it.
However, SSA construction requires non-trivial algorithms and data structures,
so it is inconvenient and wasteful for every front-end to have to reproduce this
logic.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="memory">Memory in LLVM</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The 'trick' here is that while LLVM does require all register values to be
in SSA form, it does not require (or permit) memory objects to be in SSA form.
In the example above, note that the loads from G and H are direct accesses to
G and H: they are not renamed or versioned. This differs from some other
compiler systems, which do try to version memory objects. In LLVM, instead of
encoding dataflow analysis of memory into the LLVM IR, it is handled with <a
href="../WritingAnLLVMPass.html">Analysis Passes</a> which are computed on
demand.</p>
<p>
With this in mind, the high-level idea is that we want to make a stack variable
(which lives in memory, because it is on the stack) for each mutable object in
a function. To take advantage of this trick, we need to talk about how LLVM
represents stack variables.
</p>
<p>In LLVM, all memory accesses are explicit with load/store instructions, and
it is carefully designed not to have (or need) an "address-of" operator. Notice
how the type of the @G/@H global variables is actually "i32*" even though the
variable is defined as "i32". What this means is that @G defines <em>space</em>
for an i32 in the global data area, but its <em>name</em> actually refers to the
address for that space. Stack variables work the same way, except that instead of
being declared with global variable definitions, they are declared with the
<a href="../LangRef.html#i_alloca">LLVM alloca instruction</a>:</p>
<div class="doc_code">
<pre>
define i32 @example() {
entry:
%X = alloca i32 ; type of %X is i32*.
...
%tmp = load i32* %X ; load the stack value %X from the stack.
%tmp2 = add i32 %tmp, 1 ; increment it
store i32 %tmp2, i32* %X ; store it back
...
</pre>
</div>
<p>This code shows an example of how you can declare and manipulate a stack
variable in the LLVM IR. Stack memory allocated with the alloca instruction is
fully general: you can pass the address of the stack slot to functions, you can
store it in other variables, etc. In our example above, we could rewrite the
example to use the alloca technique to avoid using a PHI node:</p>
<div class="doc_code">
<pre>
@G = weak global i32 0 ; type of @G is i32*
@H = weak global i32 0 ; type of @H is i32*
define i32 @test(i1 %Condition) {
entry:
%X = alloca i32 ; type of %X is i32*.
br i1 %Condition, label %cond_true, label %cond_false
cond_true:
%X.0 = load i32* @G
store i32 %X.0, i32* %X ; Update X
br label %cond_next
cond_false:
%X.1 = load i32* @H
store i32 %X.1, i32* %X ; Update X
br label %cond_next
cond_next:
%X.2 = load i32* %X ; Read X
ret i32 %X.2
}
</pre>
</div>
<p>With this, we have discovered a way to handle arbitrary mutable variables
without the need to create Phi nodes at all:</p>
<ol>
<li>Each mutable variable becomes a stack allocation.</li>
<li>Each read of the variable becomes a load from the stack.</li>
<li>Each update of the variable becomes a store to the stack.</li>
<li>Taking the address of a variable just uses the stack address directly.</li>
</ol>
<p>While this solution has solved our immediate problem, it introduced another
one: we have now apparently introduced a lot of stack traffic for very simple
and common operations, a major performance problem. Fortunately for us, the
LLVM optimizer has a highly-tuned optimization pass named "mem2reg" that handles
this case, promoting allocas like this into SSA registers, inserting Phi nodes
as appropriate. If you run this example through the pass, for example, you'll
get:</p>
<div class="doc_code">
<pre>
$ <b>llvm-as < example.ll | opt -mem2reg | llvm-dis</b>
@G = weak global i32 0
@H = weak global i32 0
define i32 @test(i1 %Condition) {
entry:
br i1 %Condition, label %cond_true, label %cond_false
cond_true:
%X.0 = load i32* @G
br label %cond_next
cond_false:
%X.1 = load i32* @H
br label %cond_next
cond_next:
%X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
ret i32 %X.01
}
</pre>
</div>
<p>The mem2reg pass implements the standard "iterated dominance frontier"
algorithm for constructing SSA form and has a number of optimizations that speed
up (very common) degenerate cases. The mem2reg optimization pass is the answer
to dealing with mutable variables, and we highly recommend that you depend on
it. Note that mem2reg only works on variables in certain circumstances:</p>
<ol>
<li>mem2reg is alloca-driven: it looks for allocas and if it can handle them, it
promotes them. It does not apply to global variables or heap allocations.</li>
<li>mem2reg only looks for alloca instructions in the entry block of the
function. Being in the entry block guarantees that the alloca is only executed
once, which makes analysis simpler.</li>
<li>mem2reg only promotes allocas whose uses are direct loads and stores. If
the address of the stack object is passed to a function, or if any funny pointer
arithmetic is involved, the alloca will not be promoted.</li>
<li>mem2reg only works on allocas of <a
href="../LangRef.html#t_classifications">first class</a>
values (such as pointers, scalars and vectors), and only if the array size
of the allocation is 1 (or missing in the .ll file). mem2reg is not capable of
promoting structs or arrays to registers. Note that the "scalarrepl" pass is
more powerful and can promote structs, "unions", and arrays in many cases.</li>
</ol>
<p>
All of these properties are easy to satisfy for most imperative languages, and
we'll illustrate it below with Kaleidoscope. The final question you may be
asking is: should I bother with this nonsense for my front-end? Wouldn't it be
better if I just did SSA construction directly, avoiding use of the mem2reg
optimization pass? In short, we strongly recommend that you use this technique
for building SSA form, unless there is an extremely good reason not to. Using
this technique is:</p>
<ul>
<li>Proven and well tested: llvm-gcc and clang both use this technique for local
mutable variables. As such, the most common clients of LLVM are using this to
handle a bulk of their variables. You can be sure that bugs are found fast and
fixed early.</li>
<li>Extremely Fast: mem2reg has a number of special cases that make it fast in
common cases as well as fully general. For example, it has fast-paths for
variables that are only used in a single block, variables that only have one
assignment point, good heuristics to avoid insertion of unneeded phi nodes, etc.
</li>
<li>Needed for debug info generation: <a href="../SourceLevelDebugging.html">
Debug information in LLVM</a> relies on having the address of the variable
exposed so that debug info can be attached to it. This technique dovetails
very naturally with this style of debug info.</li>
</ul>
<p>If nothing else, this makes it much easier to get your front-end up and
running, and is very simple to implement. Lets extend Kaleidoscope with mutable
variables now!
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="kalvars">Mutable Variables in
Kaleidoscope</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Now that we know the sort of problem we want to tackle, lets see what this
looks like in the context of our little Kaleidoscope language. We're going to
add two features:</p>
<ol>
<li>The ability to mutate variables with the '=' operator.</li>
<li>The ability to define new variables.</li>
</ol>
<p>While the first item is really what this is about, we only have variables
for incoming arguments as well as for induction variables, and redefining those only
goes so far :). Also, the ability to define new variables is a
useful thing regardless of whether you will be mutating them. Here's a
motivating example that shows how we could use these:</p>
<div class="doc_code">
<pre>
# Define ':' for sequencing: as a low-precedence operator that ignores operands
# and just returns the RHS.
def binary : 1 (x y) y;
# Recursive fib, we could do this before.
def fib(x)
if (x < 3) then
1
else
fib(x-1)+fib(x-2);
# Iterative fib.
def fibi(x)
<b>var a = 1, b = 1, c in</b>
(for i = 3, i < x in
<b>c = a + b</b> :
<b>a = b</b> :
<b>b = c</b>) :
b;
# Call it.
fibi(10);
</pre>
</div>
<p>
In order to mutate variables, we have to change our existing variables to use
the "alloca trick". Once we have that, we'll add our new operator, then extend
Kaleidoscope to support new variable definitions.
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="adjustments">Adjusting Existing Variables for
Mutation</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
The symbol table in Kaleidoscope is managed at code generation time by the
'<tt>named_values</tt>' map. This map currently keeps track of the LLVM
"Value*" that holds the double value for the named variable. In order to
support mutation, we need to change this slightly, so that it
<tt>named_values</tt> holds the <em>memory location</em> of the variable in
question. Note that this change is a refactoring: it changes the structure of
the code, but does not (by itself) change the behavior of the compiler. All of
these changes are isolated in the Kaleidoscope code generator.</p>
<p>
At this point in Kaleidoscope's development, it only supports variables for two
things: incoming arguments to functions and the induction variable of 'for'
loops. For consistency, we'll allow mutation of these variables in addition to
other user-defined variables. This means that these will both need memory
locations.
</p>
<p>To start our transformation of Kaleidoscope, we'll change the
<tt>named_values</tt> map so that it maps to AllocaInst* instead of Value*.
Once we do this, the C++ compiler will tell us what parts of the code we need to
update:</p>
<p><b>Note:</b> the ocaml bindings currently model both <tt>Value*</tt>s and
<tt>AllocInst*</tt>s as <tt>Llvm.llvalue</tt>s, but this may change in the
future to be more type safe.</p>
<div class="doc_code">
<pre>
let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
</pre>
</div>
<p>Also, since we will need to create these alloca's, we'll use a helper
function that ensures that the allocas are created in the entry block of the
function:</p>
<div class="doc_code">
<pre>
(* Create an alloca instruction in the entry block of the function. This
* is used for mutable variables etc. *)
let create_entry_block_alloca the_function var_name =
let builder = builder_at (instr_begin (entry_block the_function)) in
build_alloca double_type var_name builder
</pre>
</div>
<p>This funny looking code creates an <tt>Llvm.llbuilder</tt> object that is
pointing at the first instruction of the entry block. It then creates an alloca
with the expected name and returns it. Because all values in Kaleidoscope are
doubles, there is no need to pass in a type to use.</p>
<p>With this in place, the first functionality change we want to make is to
variable references. In our new scheme, variables live on the stack, so code
generating a reference to them actually needs to produce a load from the stack
slot:</p>
<div class="doc_code">
<pre>
let rec codegen_expr = function
...
| Ast.Variable name ->
let v = try Hashtbl.find named_values name with
| Not_found -> raise (Error "unknown variable name")
in
<b>(* Load the value. *)
build_load v name builder</b>
</pre>
</div>
<p>As you can see, this is pretty straightforward. Now we need to update the
things that define the variables to set up the alloca. We'll start with
<tt>codegen_expr Ast.For ...</tt> (see the <a href="#code">full code listing</a>
for the unabridged code):</p>
<div class="doc_code">
<pre>
| Ast.For (var_name, start, end_, step, body) ->
let the_function = block_parent (insertion_block builder) in
(* Create an alloca for the variable in the entry block. *)
<b>let alloca = create_entry_block_alloca the_function var_name in</b>
(* Emit the start code first, without 'variable' in scope. *)
let start_val = codegen_expr start in
<b>(* Store the value into the alloca. *)
ignore(build_store start_val alloca builder);</b>
...
(* Within the loop, the variable is defined equal to the PHI node. If it
* shadows an existing variable, we have to restore it, so save it
* now. *)
let old_val =
try Some (Hashtbl.find named_values var_name) with Not_found -> None
in
<b>Hashtbl.add named_values var_name alloca;</b>
...
(* Compute the end condition. *)
let end_cond = codegen_expr end_ in
<b>(* Reload, increment, and restore the alloca. This handles the case where
* the body of the loop mutates the variable. *)
let cur_var = build_load alloca var_name builder in
let next_var = build_add cur_var step_val "nextvar" builder in
ignore(build_store next_var alloca builder);</b>
...
</pre>
</div>
<p>This code is virtually identical to the code <a
href="OCamlLangImpl5.html#forcodegen">before we allowed mutable variables</a>.
The big difference is that we no longer have to construct a PHI node, and we use
load/store to access the variable as needed.</p>
<p>To support mutable argument variables, we need to also make allocas for them.
The code for this is also pretty simple:</p>
<div class="doc_code">
<pre>
(* Create an alloca for each argument and register the argument in the symbol
* table so that references to it will succeed. *)
let create_argument_allocas the_function proto =
let args = match proto with
| Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -> args
in
Array.iteri (fun i ai ->
let var_name = args.(i) in
(* Create an alloca for this variable. *)
let alloca = create_entry_block_alloca the_function var_name in
(* Store the initial value into the alloca. *)
ignore(build_store ai alloca builder);
(* Add arguments to variable symbol table. *)
Hashtbl.add named_values var_name alloca;
) (params the_function)
</pre>
</div>
<p>For each argument, we make an alloca, store the input value to the function
into the alloca, and register the alloca as the memory location for the
argument. This method gets invoked by <tt>Codegen.codegen_func</tt> right after
it sets up the entry block for the function.</p>
<p>The final missing piece is adding the mem2reg pass, which allows us to get
good codegen once again:</p>
<div class="doc_code">
<pre>
let main () =
...
let the_fpm = PassManager.create_function Codegen.the_module in
(* Set up the optimizer pipeline. Start with registering info about how the
* target lays out data structures. *)
TargetData.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
<b>(* Promote allocas to registers. *)
add_memory_to_register_promotion the_fpm;</b>
(* Do simple "peephole" optimizations and bit-twiddling optzn. *)
add_instruction_combining the_fpm;
(* reassociate expressions. *)
add_reassociation the_fpm;
</pre>
</div>
<p>It is interesting to see what the code looks like before and after the
mem2reg optimization runs. For example, this is the before/after code for our
recursive fib function. Before the optimization:</p>
<div class="doc_code">
<pre>
define double @fib(double %x) {
entry:
<b>%x1 = alloca double
store double %x, double* %x1
%x2 = load double* %x1</b>
%cmptmp = fcmp ult double %x2, 3.000000e+00
%booltmp = uitofp i1 %cmptmp to double
%ifcond = fcmp one double %booltmp, 0.000000e+00
br i1 %ifcond, label %then, label %else
then: ; preds = %entry
br label %ifcont
else: ; preds = %entry
<b>%x3 = load double* %x1</b>
%subtmp = fsub double %x3, 1.000000e+00
%calltmp = call double @fib( double %subtmp )
<b>%x4 = load double* %x1</b>
%subtmp5 = fsub double %x4, 2.000000e+00
%calltmp6 = call double @fib( double %subtmp5 )
%addtmp = fadd double %calltmp, %calltmp6
br label %ifcont
ifcont: ; preds = %else, %then
%iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
ret double %iftmp
}
</pre>
</div>
<p>Here there is only one variable (x, the input argument) but you can still
see the extremely simple-minded code generation strategy we are using. In the
entry block, an alloca is created, and the initial input value is stored into
it. Each reference to the variable does a reload from the stack. Also, note
that we didn't modify the if/then/else expression, so it still inserts a PHI
node. While we could make an alloca for it, it is actually easier to create a
PHI node for it, so we still just make the PHI.</p>
<p>Here is the code after the mem2reg pass runs:</p>
<div class="doc_code">
<pre>
define double @fib(double %x) {
entry:
%cmptmp = fcmp ult double <b>%x</b>, 3.000000e+00
%booltmp = uitofp i1 %cmptmp to double
%ifcond = fcmp one double %booltmp, 0.000000e+00
br i1 %ifcond, label %then, label %else
then:
br label %ifcont
else:
%subtmp = fsub double <b>%x</b>, 1.000000e+00
%calltmp = call double @fib( double %subtmp )
%subtmp5 = fsub double <b>%x</b>, 2.000000e+00
%calltmp6 = call double @fib( double %subtmp5 )
%addtmp = fadd double %calltmp, %calltmp6
br label %ifcont
ifcont: ; preds = %else, %then
%iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
ret double %iftmp
}
</pre>
</div>
<p>This is a trivial case for mem2reg, since there are no redefinitions of the
variable. The point of showing this is to calm your tension about inserting
such blatent inefficiencies :).</p>
<p>After the rest of the optimizers run, we get:</p>
<div class="doc_code">
<pre>
define double @fib(double %x) {
entry:
%cmptmp = fcmp ult double %x, 3.000000e+00
%booltmp = uitofp i1 %cmptmp to double
%ifcond = fcmp ueq double %booltmp, 0.000000e+00
br i1 %ifcond, label %else, label %ifcont
else:
%subtmp = fsub double %x, 1.000000e+00
%calltmp = call double @fib( double %subtmp )
%subtmp5 = fsub double %x, 2.000000e+00
%calltmp6 = call double @fib( double %subtmp5 )
%addtmp = fadd double %calltmp, %calltmp6
ret double %addtmp
ifcont:
ret double 1.000000e+00
}
</pre>
</div>
<p>Here we see that the simplifycfg pass decided to clone the return instruction
into the end of the 'else' block. This allowed it to eliminate some branches
and the PHI node.</p>
<p>Now that all symbol table references are updated to use stack variables,
we'll add the assignment operator.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="assignment">New Assignment Operator</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>With our current framework, adding a new assignment operator is really
simple. We will parse it just like any other binary operator, but handle it
internally (instead of allowing the user to define it). The first step is to
set a precedence:</p>
<div class="doc_code">
<pre>
let main () =
(* Install standard binary operators.
* 1 is the lowest precedence. *)
<b>Hashtbl.add Parser.binop_precedence '=' 2;</b>
Hashtbl.add Parser.binop_precedence '<' 10;
Hashtbl.add Parser.binop_precedence '+' 20;
Hashtbl.add Parser.binop_precedence '-' 20;
...
</pre>
</div>
<p>Now that the parser knows the precedence of the binary operator, it takes
care of all the parsing and AST generation. We just need to implement codegen
for the assignment operator. This looks like:</p>
<div class="doc_code">
<pre>
let rec codegen_expr = function
begin match op with
| '=' ->
(* Special case '=' because we don't want to emit the LHS as an
* expression. *)
let name =
match lhs with
| Ast.Variable name -> name
| _ -> raise (Error "destination of '=' must be a variable")
in
</pre>
</div>
<p>Unlike the rest of the binary operators, our assignment operator doesn't
follow the "emit LHS, emit RHS, do computation" model. As such, it is handled
as a special case before the other binary operators are handled. The other
strange thing is that it requires the LHS to be a variable. It is invalid to
have "(x+1) = expr" - only things like "x = expr" are allowed.
</p>
<div class="doc_code">
<pre>
(* Codegen the rhs. *)
let val_ = codegen_expr rhs in
(* Lookup the name. *)
let variable = try Hashtbl.find named_values name with
| Not_found -> raise (Error "unknown variable name")
in
ignore(build_store val_ variable builder);
val_
| _ ->
...
</pre>
</div>
<p>Once we have the variable, codegen'ing the assignment is straightforward:
we emit the RHS of the assignment, create a store, and return the computed
value. Returning a value allows for chained assignments like "X = (Y = Z)".</p>
<p>Now that we have an assignment operator, we can mutate loop variables and
arguments. For example, we can now run code like this:</p>
<div class="doc_code">
<pre>
# Function to print a double.
extern printd(x);
# Define ':' for sequencing: as a low-precedence operator that ignores operands
# and just returns the RHS.
def binary : 1 (x y) y;
def test(x)
printd(x) :
x = 4 :
printd(x);
test(123);
</pre>
</div>
<p>When run, this example prints "123" and then "4", showing that we did
actually mutate the value! Okay, we have now officially implemented our goal:
getting this to work requires SSA construction in the general case. However,
to be really useful, we want the ability to define our own local variables, lets
add this next!
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="localvars">User-defined Local
Variables</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Adding var/in is just like any other other extensions we made to
Kaleidoscope: we extend the lexer, the parser, the AST and the code generator.
The first step for adding our new 'var/in' construct is to extend the lexer.
As before, this is pretty trivial, the code looks like this:</p>
<div class="doc_code">
<pre>
type token =
...
<b>(* var definition *)
| Var</b>
...
and lex_ident buffer = parser
...
| "in" -> [< 'Token.In; stream >]
| "binary" -> [< 'Token.Binary; stream >]
| "unary" -> [< 'Token.Unary; stream >]
<b>| "var" -> [< 'Token.Var; stream >]</b>
...
</pre>
</div>
<p>The next step is to define the AST node that we will construct. For var/in,
it looks like this:</p>
<div class="doc_code">
<pre>
type expr =
...
(* variant for var/in. *)
| Var of (string * expr option) array * expr
...
</pre>
</div>
<p>var/in allows a list of names to be defined all at once, and each name can
optionally have an initializer value. As such, we capture this information in
the VarNames vector. Also, var/in has a body, this body is allowed to access
the variables defined by the var/in.</p>
<p>With this in place, we can define the parser pieces. The first thing we do
is add it as a primary expression:</p>
<div class="doc_code">
<pre>
(* primary
* ::= identifier
* ::= numberexpr
* ::= parenexpr
* ::= ifexpr
* ::= forexpr
<b>* ::= varexpr</b> *)
let rec parse_primary = parser
...
<b>(* varexpr
* ::= 'var' identifier ('=' expression?
* (',' identifier ('=' expression)?)* 'in' expression *)
| [< 'Token.Var;
(* At least one variable name is required. *)
'Token.Ident id ?? "expected identifier after var";
init=parse_var_init;
var_names=parse_var_names [(id, init)];
(* At this point, we have to have 'in'. *)
'Token.In ?? "expected 'in' keyword after 'var'";
body=parse_expr >] ->
Ast.Var (Array.of_list (List.rev var_names), body)</b>
...
and parse_var_init = parser
(* read in the optional initializer. *)
| [< 'Token.Kwd '='; e=parse_expr >] -> Some e
| [< >] -> None
and parse_var_names accumulator = parser
| [< 'Token.Kwd ',';
'Token.Ident id ?? "expected identifier list after var";
init=parse_var_init;
e=parse_var_names ((id, init) :: accumulator) >] -> e
| [< >] -> accumulator
</pre>
</div>
<p>Now that we can parse and represent the code, we need to support emission of
LLVM IR for it. This code starts out with:</p>
<div class="doc_code">
<pre>
let rec codegen_expr = function
...
| Ast.Var (var_names, body)
let old_bindings = ref [] in
let the_function = block_parent (insertion_block builder) in
(* Register all variables and emit their initializer. *)
Array.iter (fun (var_name, init) ->
</pre>
</div>
<p>Basically it loops over all the variables, installing them one at a time.
For each variable we put into the symbol table, we remember the previous value
that we replace in OldBindings.</p>
<div class="doc_code">
<pre>
(* Emit the initializer before adding the variable to scope, this
* prevents the initializer from referencing the variable itself, and
* permits stuff like this:
* var a = 1 in
* var a = a in ... # refers to outer 'a'. *)
let init_val =
match init with
| Some init -> codegen_expr init
(* If not specified, use 0.0. *)
| None -> const_float double_type 0.0
in
let alloca = create_entry_block_alloca the_function var_name in
ignore(build_store init_val alloca builder);
(* Remember the old variable binding so that we can restore the binding
* when we unrecurse. *)
begin
try
let old_value = Hashtbl.find named_values var_name in
old_bindings := (var_name, old_value) :: !old_bindings;
with Not_found > ()
end;
(* Remember this binding. *)
Hashtbl.add named_values var_name alloca;
) var_names;
</pre>
</div>
<p>There are more comments here than code. The basic idea is that we emit the
initializer, create the alloca, then update the symbol table to point to it.
Once all the variables are installed in the symbol table, we evaluate the body
of the var/in expression:</p>
<div class="doc_code">
<pre>
(* Codegen the body, now that all vars are in scope. *)
let body_val = codegen_expr body in
</pre>
</div>
<p>Finally, before returning, we restore the previous variable bindings:</p>
<div class="doc_code">
<pre>
(* Pop all our variables from scope. *)
List.iter (fun (var_name, old_value) ->
Hashtbl.add named_values var_name old_value
) !old_bindings;
(* Return the body computation. *)
body_val
</pre>
</div>
<p>The end result of all of this is that we get properly scoped variable
definitions, and we even (trivially) allow mutation of them :).</p>
<p>With this, we completed what we set out to do. Our nice iterative fib
example from the intro compiles and runs just fine. The mem2reg pass optimizes
all of our stack variables into SSA registers, inserting PHI nodes where needed,
and our front-end remains simple: no "iterated dominance frontier" computation
anywhere in sight.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="code">Full Code Listing</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
Here is the complete code listing for our running example, enhanced with mutable
variables and var/in support. To build this example, use:
</p>
<div class="doc_code">
<pre>
# Compile
ocamlbuild toy.byte
# Run
./toy.byte
</pre>
</div>
<p>Here is the code:</p>
<dl>
<dt>_tags:</dt>
<dd class="doc_code">
<pre>
<{lexer,parser}.ml>: use_camlp4, pp(camlp4of)
<*.{byte,native}>: g++, use_llvm, use_llvm_analysis
<*.{byte,native}>: use_llvm_executionengine, use_llvm_target
<*.{byte,native}>: use_llvm_scalar_opts, use_bindings
</pre>
</dd>
<dt>myocamlbuild.ml:</dt>
<dd class="doc_code">
<pre>
open Ocamlbuild_plugin;;
ocaml_lib ~extern:true "llvm";;
ocaml_lib ~extern:true "llvm_analysis";;
ocaml_lib ~extern:true "llvm_executionengine";;
ocaml_lib ~extern:true "llvm_target";;
ocaml_lib ~extern:true "llvm_scalar_opts";;
flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"; A"-cclib"; A"-rdynamic"]);;
dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
</pre>
</dd>
<dt>token.ml:</dt>
<dd class="doc_code">
<pre>
(*===----------------------------------------------------------------------===
* Lexer Tokens
*===----------------------------------------------------------------------===*)
(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
* these others for known things. *)
type token =
(* commands *)
| Def | Extern
(* primary *)
| Ident of string | Number of float
(* unknown *)
| Kwd of char
(* control *)
| If | Then | Else
| For | In
(* operators *)
| Binary | Unary
(* var definition *)
| Var
</pre>
</dd>
<dt>lexer.ml:</dt>
<dd class="doc_code">
<pre>
(*===----------------------------------------------------------------------===
* Lexer
*===----------------------------------------------------------------------===*)
let rec lex = parser
(* Skip any whitespace. *)
| [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
(* identifier: [a-zA-Z][a-zA-Z0-9] *)
| [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_ident buffer stream
(* number: [0-9.]+ *)
| [< ' ('0' .. '9' as c); stream >] ->
let buffer = Buffer.create 1 in
Buffer.add_char buffer c;
lex_number buffer stream
(* Comment until end of line. *)
| [< ' ('#'); stream >] ->
lex_comment stream
(* Otherwise, just return the character as its ascii value. *)
| [< 'c; stream >] ->
[< 'Token.Kwd c; lex stream >]
(* end of stream. *)
| [< >] -> [< >]
and lex_number buffer = parser
| [< ' ('0' .. '9' | '.' as c); stream >] ->
Buffer.add_char buffer c;
lex_number buffer stream
| [< stream=lex >] ->
[< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
and lex_ident buffer = parser
| [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
Buffer.add_char buffer c;
lex_ident buffer stream
| [< stream=lex >] ->
match Buffer.contents buffer with
| "def" -> [< 'Token.Def; stream >]
| "extern" -> [< 'Token.Extern; stream >]
| "if" -> [< 'Token.If; stream >]
| "then" -> [< 'Token.Then; stream >]
| "else" -> [< 'Token.Else; stream >]
| "for" -> [< 'Token.For; stream >]
| "in" -> [< 'Token.In; stream >]
| "binary" -> [< 'Token.Binary; stream >]
| "unary" -> [< 'Token.Unary; stream >]
| "var" -> [< 'Token.Var; stream >]
| id -> [< 'Token.Ident id; stream >]
and lex_comment = parser
| [< ' ('\n'); stream=lex >] -> stream
| [< 'c; e=lex_comment >] -> e
| [< >] -> [< >]
</pre>
</dd>
<dt>ast.ml:</dt>
<dd class="doc_code">
<pre>
(*===----------------------------------------------------------------------===
* Abstract Syntax Tree (aka Parse Tree)
*===----------------------------------------------------------------------===*)
(* expr - Base type for all expression nodes. *)
type expr =
(* variant for numeric literals like "1.0". *)
| Number of float
(* variant for referencing a variable, like "a". *)
| Variable of string
(* variant for a unary operator. *)
| Unary of char * expr
(* variant for a binary operator. *)
| Binary of char * expr * expr
(* variant for function calls. *)
| Call of string * expr array
(* variant for if/then/else. *)
| If of expr * expr * expr
(* variant for for/in. *)
| For of string * expr * expr * expr option * expr
(* variant for var/in. *)
| Var of (string * expr option) array * expr
(* proto - This type represents the "prototype" for a function, which captures
* its name, and its argument names (thus implicitly the number of arguments the
* function takes). *)
type proto =
| Prototype of string * string array
| BinOpPrototype of string * string array * int
(* func - This type represents a function definition itself. *)
type func = Function of proto * expr
</pre>
</dd>
<dt>parser.ml:</dt>
<dd class="doc_code">
<pre>
(*===---------------------------------------------------------------------===
* Parser
*===---------------------------------------------------------------------===*)
(* binop_precedence - This holds the precedence for each binary operator that is
* defined *)
let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
(* precedence - Get the precedence of the pending binary operator token. *)
let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
(* primary
* ::= identifier
* ::= numberexpr
* ::= parenexpr
* ::= ifexpr
* ::= forexpr
* ::= varexpr *)
let rec parse_primary = parser
(* numberexpr ::= number *)
| [< 'Token.Number n >] -> Ast.Number n
(* parenexpr ::= '(' expression ')' *)
| [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
(* identifierexpr
* ::= identifier
* ::= identifier '(' argumentexpr ')' *)
| [< 'Token.Ident id; stream >] ->
let rec parse_args accumulator = parser
| [< e=parse_expr; stream >] ->
begin parser
| [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
| [< >] -> e :: accumulator
end stream
| [< >] -> accumulator
in
let rec parse_ident id = parser
(* Call. *)
| [< 'Token.Kwd '(';
args=parse_args [];
'Token.Kwd ')' ?? "expected ')'">] ->
Ast.Call (id, Array.of_list (List.rev args))
(* Simple variable ref. *)
| [< >] -> Ast.Variable id
in
parse_ident id stream
(* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
| [< 'Token.If; c=parse_expr;
'Token.Then ?? "expected 'then'"; t=parse_expr;
'Token.Else ?? "expected 'else'"; e=parse_expr >] ->
Ast.If (c, t, e)
(* forexpr
::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
| [< 'Token.For;
'Token.Ident id ?? "expected identifier after for";
'Token.Kwd '=' ?? "expected '=' after for";
stream >] ->
begin parser
| [<
start=parse_expr;
'Token.Kwd ',' ?? "expected ',' after for";
end_=parse_expr;
stream >] ->
let step =
begin parser
| [< 'Token.Kwd ','; step=parse_expr >] -> Some step
| [< >] -> None
end stream
in
begin parser
| [< 'Token.In; body=parse_expr >] ->
Ast.For (id, start, end_, step, body)
| [< >] ->
raise (Stream.Error "expected 'in' after for")
end stream
| [< >] ->
raise (Stream.Error "expected '=' after for")
end stream
(* varexpr
* ::= 'var' identifier ('=' expression?
* (',' identifier ('=' expression)?)* 'in' expression *)
| [< 'Token.Var;
(* At least one variable name is required. *)
'Token.Ident id ?? "expected identifier after var";
init=parse_var_init;
var_names=parse_var_names [(id, init)];
(* At this point, we have to have 'in'. *)
'Token.In ?? "expected 'in' keyword after 'var'";
body=parse_expr >] ->
Ast.Var (Array.of_list (List.rev var_names), body)
| [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
(* unary
* ::= primary
* ::= '!' unary *)
and parse_unary = parser
(* If this is a unary operator, read it. *)
| [< 'Token.Kwd op when op != '(' && op != ')'; operand=parse_expr >] ->
Ast.Unary (op, operand)
(* If the current token is not an operator, it must be a primary expr. *)
| [< stream >] -> parse_primary stream
(* binoprhs
* ::= ('+' primary)* *)
and parse_bin_rhs expr_prec lhs stream =
match Stream.peek stream with
(* If this is a binop, find its precedence. *)
| Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
let token_prec = precedence c in
(* If this is a binop that binds at least as tightly as the current binop,
* consume it, otherwise we are done. *)
if token_prec < expr_prec then lhs else begin
(* Eat the binop. *)
Stream.junk stream;
(* Parse the primary expression after the binary operator. *)
let rhs = parse_unary stream in
(* Okay, we know this is a binop. *)
let rhs =
match Stream.peek stream with
| Some (Token.Kwd c2) ->
(* If BinOp binds less tightly with rhs than the operator after
* rhs, let the pending operator take rhs as its lhs. *)
let next_prec = precedence c2 in
if token_prec < next_prec
then parse_bin_rhs (token_prec + 1) rhs stream
else rhs
| _ -> rhs
in
(* Merge lhs/rhs. *)
let lhs = Ast.Binary (c, lhs, rhs) in
parse_bin_rhs expr_prec lhs stream
end
| _ -> lhs
and parse_var_init = parser
(* read in the optional initializer. *)
| [< 'Token.Kwd '='; e=parse_expr >] -> Some e
| [< >] -> None
and parse_var_names accumulator = parser
| [< 'Token.Kwd ',';
'Token.Ident id ?? "expected identifier list after var";
init=parse_var_init;
e=parse_var_names ((id, init) :: accumulator) >] -> e
| [< >] -> accumulator
(* expression
* ::= primary binoprhs *)
and parse_expr = parser
| [< lhs=parse_unary; stream >] -> parse_bin_rhs 0 lhs stream
(* prototype
* ::= id '(' id* ')'
* ::= binary LETTER number? (id, id)
* ::= unary LETTER number? (id) *)
let parse_prototype =
let rec parse_args accumulator = parser
| [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
| [< >] -> accumulator
in
let parse_operator = parser
| [< 'Token.Unary >] -> "unary", 1
| [< 'Token.Binary >] -> "binary", 2
in
let parse_binary_precedence = parser
| [< 'Token.Number n >] -> int_of_float n
| [< >] -> 30
in
parser
| [< 'Token.Ident id;
'Token.Kwd '(' ?? "expected '(' in prototype";
args=parse_args [];
'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
(* success. *)
Ast.Prototype (id, Array.of_list (List.rev args))
| [< (prefix, kind)=parse_operator;
'Token.Kwd op ?? "expected an operator";
(* Read the precedence if present. *)
binary_precedence=parse_binary_precedence;
'Token.Kwd '(' ?? "expected '(' in prototype";
args=parse_args [];
'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
let name = prefix ^ (String.make 1 op) in
let args = Array.of_list (List.rev args) in
(* Verify right number of arguments for operator. *)
if Array.length args != kind
then raise (Stream.Error "invalid number of operands for operator")
else
if kind == 1 then
Ast.Prototype (name, args)
else
Ast.BinOpPrototype (name, args, binary_precedence)
| [< >] ->
raise (Stream.Error "expected function name in prototype")
(* definition ::= 'def' prototype expression *)
let parse_definition = parser
| [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
Ast.Function (p, e)
(* toplevelexpr ::= expression *)
let parse_toplevel = parser
| [< e=parse_expr >] ->
(* Make an anonymous proto. *)
Ast.Function (Ast.Prototype ("", [||]), e)
(* external ::= 'extern' prototype *)
let parse_extern = parser
| [< 'Token.Extern; e=parse_prototype >] -> e
</pre>
</dd>
<dt>codegen.ml:</dt>
<dd class="doc_code">
<pre>
(*===----------------------------------------------------------------------===
* Code Generation
*===----------------------------------------------------------------------===*)
open Llvm
exception Error of string
let context = global_context ()
let the_module = create_module context "my cool jit"
let builder = builder context
let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
let double_type = double_type context
(* Create an alloca instruction in the entry block of the function. This
* is used for mutable variables etc. *)
let create_entry_block_alloca the_function var_name =
let builder = builder_at context (instr_begin (entry_block the_function)) in
build_alloca double_type var_name builder
let rec codegen_expr = function
| Ast.Number n -> const_float double_type n
| Ast.Variable name ->
let v = try Hashtbl.find named_values name with
| Not_found -> raise (Error "unknown variable name")
in
(* Load the value. *)
build_load v name builder
| Ast.Unary (op, operand) ->
let operand = codegen_expr operand in
let callee = "unary" ^ (String.make 1 op) in
let callee =
match lookup_function callee the_module with
| Some callee -> callee
| None -> raise (Error "unknown unary operator")
in
build_call callee [|operand|] "unop" builder
| Ast.Binary (op, lhs, rhs) ->
begin match op with
| '=' ->
(* Special case '=' because we don't want to emit the LHS as an
* expression. *)
let name =
match lhs with
| Ast.Variable name -> name
| _ -> raise (Error "destination of '=' must be a variable")
in
(* Codegen the rhs. *)
let val_ = codegen_expr rhs in
(* Lookup the name. *)
let variable = try Hashtbl.find named_values name with
| Not_found -> raise (Error "unknown variable name")
in
ignore(build_store val_ variable builder);
val_
| _ ->
let lhs_val = codegen_expr lhs in
let rhs_val = codegen_expr rhs in
begin
match op with
| '+' -> build_add lhs_val rhs_val "addtmp" builder
| '-' -> build_sub lhs_val rhs_val "subtmp" builder
| '*' -> build_mul lhs_val rhs_val "multmp" builder
| '<' ->
(* Convert bool 0/1 to double 0.0 or 1.0 *)
let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
build_uitofp i double_type "booltmp" builder
| _ ->
(* If it wasn't a builtin binary operator, it must be a user defined
* one. Emit a call to it. *)
let callee = "binary" ^ (String.make 1 op) in
let callee =
match lookup_function callee the_module with
| Some callee -> callee
| None -> raise (Error "binary operator not found!")
in
build_call callee [|lhs_val; rhs_val|] "binop" builder
end
end
| Ast.Call (callee, args) ->
(* Look up the name in the module table. *)
let callee =
match lookup_function callee the_module with
| Some callee -> callee
| None -> raise (Error "unknown function referenced")
in
let params = params callee in
(* If argument mismatch error. *)
if Array.length params == Array.length args then () else
raise (Error "incorrect # arguments passed");
let args = Array.map codegen_expr args in
build_call callee args "calltmp" builder
| Ast.If (cond, then_, else_) ->
let cond = codegen_expr cond in
(* Convert condition to a bool by comparing equal to 0.0 *)
let zero = const_float double_type 0.0 in
let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
(* Grab the first block so that we might later add the conditional branch
* to it at the end of the function. *)
let start_bb = insertion_block builder in
let the_function = block_parent start_bb in
let then_bb = append_block context "then" the_function in
(* Emit 'then' value. *)
position_at_end then_bb builder;
let then_val = codegen_expr then_ in
(* Codegen of 'then' can change the current block, update then_bb for the
* phi. We create a new name because one is used for the phi node, and the
* other is used for the conditional branch. *)
let new_then_bb = insertion_block builder in
(* Emit 'else' value. *)
let else_bb = append_block context "else" the_function in
position_at_end else_bb builder;
let else_val = codegen_expr else_ in
(* Codegen of 'else' can change the current block, update else_bb for the
* phi. *)
let new_else_bb = insertion_block builder in
(* Emit merge block. *)
let merge_bb = append_block context "ifcont" the_function in
position_at_end merge_bb builder;
let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
let phi = build_phi incoming "iftmp" builder in
(* Return to the start block to add the conditional branch. *)
position_at_end start_bb builder;
ignore (build_cond_br cond_val then_bb else_bb builder);
(* Set a unconditional branch at the end of the 'then' block and the
* 'else' block to the 'merge' block. *)
position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
(* Finally, set the builder to the end of the merge block. *)
position_at_end merge_bb builder;
phi
| Ast.For (var_name, start, end_, step, body) ->
(* Output this as:
* var = alloca double
* ...
* start = startexpr
* store start -> var
* goto loop
* loop:
* ...
* bodyexpr
* ...
* loopend:
* step = stepexpr
* endcond = endexpr
*
* curvar = load var
* nextvar = curvar + step
* store nextvar -> var
* br endcond, loop, endloop
* outloop: *)
let the_function = block_parent (insertion_block builder) in
(* Create an alloca for the variable in the entry block. *)
let alloca = create_entry_block_alloca the_function var_name in
(* Emit the start code first, without 'variable' in scope. *)
let start_val = codegen_expr start in
(* Store the value into the alloca. *)
ignore(build_store start_val alloca builder);
(* Make the new basic block for the loop header, inserting after current
* block. *)
let loop_bb = append_block context "loop" the_function in
(* Insert an explicit fall through from the current block to the
* loop_bb. *)
ignore (build_br loop_bb builder);
(* Start insertion in loop_bb. *)
position_at_end loop_bb builder;
(* Within the loop, the variable is defined equal to the PHI node. If it
* shadows an existing variable, we have to restore it, so save it
* now. *)
let old_val =
try Some (Hashtbl.find named_values var_name) with Not_found -> None
in
Hashtbl.add named_values var_name alloca;
(* Emit the body of the loop. This, like any other expr, can change the
* current BB. Note that we ignore the value computed by the body, but
* don't allow an error *)
ignore (codegen_expr body);
(* Emit the step value. *)
let step_val =
match step with
| Some step -> codegen_expr step
(* If not specified, use 1.0. *)
| None -> const_float double_type 1.0
in
(* Compute the end condition. *)
let end_cond = codegen_expr end_ in
(* Reload, increment, and restore the alloca. This handles the case where
* the body of the loop mutates the variable. *)
let cur_var = build_load alloca var_name builder in
let next_var = build_add cur_var step_val "nextvar" builder in
ignore(build_store next_var alloca builder);
(* Convert condition to a bool by comparing equal to 0.0. *)
let zero = const_float double_type 0.0 in
let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
(* Create the "after loop" block and insert it. *)
let after_bb = append_block context "afterloop" the_function in
(* Insert the conditional branch into the end of loop_end_bb. *)
ignore (build_cond_br end_cond loop_bb after_bb builder);
(* Any new code will be inserted in after_bb. *)
position_at_end after_bb builder;
(* Restore the unshadowed variable. *)
begin match old_val with
| Some old_val -> Hashtbl.add named_values var_name old_val
| None -> ()
end;
(* for expr always returns 0.0. *)
const_null double_type
| Ast.Var (var_names, body) ->
let old_bindings = ref [] in
let the_function = block_parent (insertion_block builder) in
(* Register all variables and emit their initializer. *)
Array.iter (fun (var_name, init) ->
(* Emit the initializer before adding the variable to scope, this
* prevents the initializer from referencing the variable itself, and
* permits stuff like this:
* var a = 1 in
* var a = a in ... # refers to outer 'a'. *)
let init_val =
match init with
| Some init -> codegen_expr init
(* If not specified, use 0.0. *)
| None -> const_float double_type 0.0
in
let alloca = create_entry_block_alloca the_function var_name in
ignore(build_store init_val alloca builder);
(* Remember the old variable binding so that we can restore the binding
* when we unrecurse. *)
begin
try
let old_value = Hashtbl.find named_values var_name in
old_bindings := (var_name, old_value) :: !old_bindings;
with Not_found -> ()
end;
(* Remember this binding. *)
Hashtbl.add named_values var_name alloca;
) var_names;
(* Codegen the body, now that all vars are in scope. *)
let body_val = codegen_expr body in
(* Pop all our variables from scope. *)
List.iter (fun (var_name, old_value) ->
Hashtbl.add named_values var_name old_value
) !old_bindings;
(* Return the body computation. *)
body_val
let codegen_proto = function
| Ast.Prototype (name, args) | Ast.BinOpPrototype (name, args, _) ->
(* Make the function type: double(double,double) etc. *)
let doubles = Array.make (Array.length args) double_type in
let ft = function_type double_type doubles in
let f =
match lookup_function name the_module with
| None -> declare_function name ft the_module
(* If 'f' conflicted, there was already something named 'name'. If it
* has a body, don't allow redefinition or reextern. *)
| Some f ->
(* If 'f' already has a body, reject this. *)
if block_begin f <> At_end f then
raise (Error "redefinition of function");
(* If 'f' took a different number of arguments, reject. *)
if element_type (type_of f) <> ft then
raise (Error "redefinition of function with different # args");
f
in
(* Set names for all arguments. *)
Array.iteri (fun i a ->
let n = args.(i) in
set_value_name n a;
Hashtbl.add named_values n a;
) (params f);
f
(* Create an alloca for each argument and register the argument in the symbol
* table so that references to it will succeed. *)
let create_argument_allocas the_function proto =
let args = match proto with
| Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -> args
in
Array.iteri (fun i ai ->
let var_name = args.(i) in
(* Create an alloca for this variable. *)
let alloca = create_entry_block_alloca the_function var_name in
(* Store the initial value into the alloca. *)
ignore(build_store ai alloca builder);
(* Add arguments to variable symbol table. *)
Hashtbl.add named_values var_name alloca;
) (params the_function)
let codegen_func the_fpm = function
| Ast.Function (proto, body) ->
Hashtbl.clear named_values;
let the_function = codegen_proto proto in
(* If this is an operator, install it. *)
begin match proto with
| Ast.BinOpPrototype (name, args, prec) ->
let op = name.[String.length name - 1] in
Hashtbl.add Parser.binop_precedence op prec;
| _ -> ()
end;
(* Create a new basic block to start insertion into. *)
let bb = append_block context "entry" the_function in
position_at_end bb builder;
try
(* Add all arguments to the symbol table and create their allocas. *)
create_argument_allocas the_function proto;
let ret_val = codegen_expr body in
(* Finish off the function. *)
let _ = build_ret ret_val builder in
(* Validate the generated code, checking for consistency. *)
Llvm_analysis.assert_valid_function the_function;
(* Optimize the function. *)
let _ = PassManager.run_function the_function the_fpm in
the_function
with e ->
delete_function the_function;
raise e
</pre>
</dd>
<dt>toplevel.ml:</dt>
<dd class="doc_code">
<pre>
(*===----------------------------------------------------------------------===
* Top-Level parsing and JIT Driver
*===----------------------------------------------------------------------===*)
open Llvm
open Llvm_executionengine
(* top ::= definition | external | expression | ';' *)
let rec main_loop the_fpm the_execution_engine stream =
match Stream.peek stream with
| None -> ()
(* ignore top-level semicolons. *)
| Some (Token.Kwd ';') ->
Stream.junk stream;
main_loop the_fpm the_execution_engine stream
| Some token ->
begin
try match token with
| Token.Def ->
let e = Parser.parse_definition stream in
print_endline "parsed a function definition.";
dump_value (Codegen.codegen_func the_fpm e);
| Token.Extern ->
let e = Parser.parse_extern stream in
print_endline "parsed an extern.";
dump_value (Codegen.codegen_proto e);
| _ ->
(* Evaluate a top-level expression into an anonymous function. *)
let e = Parser.parse_toplevel stream in
print_endline "parsed a top-level expr";
let the_function = Codegen.codegen_func the_fpm e in
dump_value the_function;
(* JIT the function, returning a function pointer. *)
let result = ExecutionEngine.run_function the_function [||]
the_execution_engine in
print_string "Evaluated to ";
print_float (GenericValue.as_float Codegen.double_type result);
print_newline ();
with Stream.Error s | Codegen.Error s ->
(* Skip token for error recovery. *)
Stream.junk stream;
print_endline s;
end;
print_string "ready> "; flush stdout;
main_loop the_fpm the_execution_engine stream
</pre>
</dd>
<dt>toy.ml:</dt>
<dd class="doc_code">
<pre>
(*===----------------------------------------------------------------------===
* Main driver code.
*===----------------------------------------------------------------------===*)
open Llvm
open Llvm_executionengine
open Llvm_target
open Llvm_scalar_opts
let main () =
ignore (initialize_native_target ());
(* Install standard binary operators.
* 1 is the lowest precedence. *)
Hashtbl.add Parser.binop_precedence '=' 2;
Hashtbl.add Parser.binop_precedence '<' 10;
Hashtbl.add Parser.binop_precedence '+' 20;
Hashtbl.add Parser.binop_precedence '-' 20;
Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *)
(* Prime the first token. *)
print_string "ready> "; flush stdout;
let stream = Lexer.lex (Stream.of_channel stdin) in
(* Create the JIT. *)
let the_execution_engine = ExecutionEngine.create Codegen.the_module in
let the_fpm = PassManager.create_function Codegen.the_module in
(* Set up the optimizer pipeline. Start with registering info about how the
* target lays out data structures. *)
TargetData.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
(* Promote allocas to registers. *)
add_memory_to_register_promotion the_fpm;
(* Do simple "peephole" optimizations and bit-twiddling optzn. *)
add_instruction_combination the_fpm;
(* reassociate expressions. *)
add_reassociation the_fpm;
(* Eliminate Common SubExpressions. *)
add_gvn the_fpm;
(* Simplify the control flow graph (deleting unreachable blocks, etc). *)
add_cfg_simplification the_fpm;
ignore (PassManager.initialize the_fpm);
(* Run the main "interpreter loop" now. *)
Toplevel.main_loop the_fpm the_execution_engine stream;
(* Print out all the generated code. *)
dump_module Codegen.the_module
;;
main ()
</pre>
</dd>
<dt>bindings.c</dt>
<dd class="doc_code">
<pre>
#include <stdio.h>
/* putchard - putchar that takes a double and returns 0. */
extern double putchard(double X) {
putchar((char)X);
return 0;
}
/* printd - printf that takes a double prints it as "%f\n", returning 0. */
extern double printd(double X) {
printf("%f\n", X);
return 0;
}
</pre>
</dd>
</dl>
<a href="LangImpl8.html">Next: Conclusion and other useful LLVM tidbits</a>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
<a href="http://validator.w3.org/check/referer"><img
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
<a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
Last modified: $Date$
</address>
</body>
</html>
|
